The present application is related to co-pending patent application entitled xe2x80x9cA Method for Determining the Optimum Observer Heading Change in Bearings-Only Passive Emitter Trackingxe2x80x9d (Docket No. BD-01-145 (3351-068)) and assigned to the instant assignee and filed on even date herewith and is hereby incorporated by reference into this specification in its entirety.
The present invention relates generally to Electronic Surveillance Measures (ESM) intercept receivers, and more particularly, to a method for passively estimating an emitter""s position and velocity using bearings-only without requiring observer acceleration.
ESM intercept receivers are used to collect radar data, and in particular, to perform a pulse parameter measurement function. Pulse parameter measurements are used to type or xe2x80x9cfingerprintxe2x80x9d radar systems. The measurements include the traditional parameters such as pulse time of arrival (TOA), pulse width (PW), amplitude, and frequency, and also pulse internals, such as modulation, as is known to persons of skill in the art.
ESM systems also typically measure emitter bearing in azimuth and, less commonly, elevation. The emitter azimuth or bearing is used to sort the many interleaved pulses from diverse radars obtained in a single wideband receiver dwell. These bearings can also be utilized across several dwells to support a significant additional capability: passive estimation of a moving emitter""s range and velocity.
As described by Fogel and Gavish, xe2x80x9cNth-Order Dynamics Target Observability from Angle Measurementsxe2x80x9d, IEEE Transactions on Aerospace and Electronic Systems, AES-24, 3 (May 1988), conventional bearings-only passive emitter tracking requires the observer to maneuver during the sequence of receiver dwells used to collect the angle measurements. FIG. 1A illustrates an observer maneuver for an important special case of a constant velocity emitter. With reference to FIG. 1A, an observer 100 flies a constant velocity track 101 between a point where a first bearing cut 102 and a last bearing cut 103 was made. An infinite number of target tracks could produce a measured azimuth change. Examples of these ambiguous tracks are indicated by reference numerals 104, 105 and 106.
Technically, the target track is said to be xe2x80x9cunobservablexe2x80x9d (Kalman, Ho and Narenda, xe2x80x9cControllability of Linear Dynamical Systems,xe2x80x9d Contributions to Differential Equations, Vol I, McMillan, New York, 1961). As Fogel and Gavish show, obtaining observability in this case requires an observer acceleration, as illustrated in FIG. 1B. An observer 109 changes heading and flies a new track 107. Now only emitter constant velocity track 108, the extension of track 105, fits the three lines-of-bearing.
Because emitters predominantly follow piecewise constant tracks, observers typically fly constant velocity legs of short duration to estimate range, speed and heading. Thus, prior methods of bearings-only target tracking, such as the one described by U.S. Pat. No. 5,877,998 to Aidala, et al. entitled xe2x80x9cRecursive Method for Target Motion Analysis,xe2x80x9d emphasize such observer kinematics. Disadvantageously, obtaining a range estimate using the above method requires an observer to execute a turn. That is, the target is located only after the second leg has begun. Hence, Aidala et al. notes it is an object of his invention xe2x80x9cto provide an improved method . . . for providing range estimates as soon as two measurement legs of data become available.xe2x80x9d
Requiring the observer to maneuver in order to locate a moving emitter is very limiting, especially for aircraft. Since the emitter""s range and velocity are not known before the maneuver, performing the maneuver can easily put the observer in an undesirable tactical position relative to the emitting platform. Also such maneuvers require aileron and other control surface deflections, and significant bank angles, which increase the observer""s radar cross section. Hence, especially for stealth aircraft, these required turns can be extremely detrimental.
Also, deriving range, speed and heading from bearings alone does not utilize all the information ESM systems typically extract from pulse measurements of a target. These extracted parameters may allow the ESM system to categorize both the target aircraft type and overall mission. They may also allow determination of the aircraft intent during the pulse collection period. Once target aircraft platform type and intent are known it is possible to bracket target speeds within a very narrow range. Such bracketing is possible because aircraft generally do not travel at arbitrary speeds within their flight envelope. Aircraft optimize speed to match the requirements of their particular mission.
Examples of typical aircraft missions include surveillance, escort and intercept. These missions involve cruise, loiter, supersonic dash, and missile launch flight regimes. Performance in each of these regimes is determined, for a given altitude, weight and configuration, by the particular aircraft""s thrust or power available, Pa, and power required Pr As shown in aerodynamics textbooks, e.g. D. P. Raymer""s xe2x80x9cAircraft Design: A Conceptual Approach,xe2x80x9d American Institute of Aeronautics and Astronautics, Washington, D.C. 1989, Pr depends on the lift and drag coefficients CD and CL specific to the aircraft airframe.
FIG. 2 illustrates some of the discrete performance speeds determined from Pa and Pr curves for a turbojet aircraft type at a fixed altitude and weight. Raymer describes the use of such Pa-Pr curves in determining best aircraft speed for a given performance requirement. Although jet aircraft performance will be used as examples here, piston and turboprop results are entirely analogous. The minimum value at Pr 200 gives the speed 203 requiring minimum thrust. Intersection of Pa and Pr 201 gives the speed at which the aircraft stalls, while intersection 202 gives the maximum speed attainable. The maximum endurance, or longest time-in-the-air speed is indicated by reference numeral 203, which is also the speed giving the maximum lift-to-drag ratio, i.e. the maximum L/Dmax. The maximum range speed is located at point 204, which is also the speed, at any altitude, weight and configuration, for which {square root over (CL)}/CD max is maximum. Other discrete speeds such as for best rate-of-climb are similarly determined for a particular aircraft type depending on the aircraft performance curve.
Thus, a need exists in the art for a method for passively estimating an emitter""s position and velocity using bearings only that does not require observer acceleration.